The present invention relates to a method of manufacturing calcium carbide from powdered lime/limestone.
Calcium carbide has been manufactured since the turn of the century and has been used as raw material in the heavy chemical industry for a long time, e.g. for producing lime nitrogen and PVC. The significance of carbide as a chemical raw material has decreased but it is used to a great extent in the production of acetylene gas for the engineering and shipbuilding industries. Over the last couple of decades more and more carbide is being used in the iron and steel industry for desulphurization purposes.
Calcium carbide is manufactured in electric arc furnaces, primarily closed furnaces equipped with Soderberg electrodes. Burnt lime, coke and anthracite are used as raw materials. The raw materials must have a certain particle size (ca. 5-40 mm) to give the correct porosity in the bed and correct electrical conditions in the charge. The raw materials must therefore first be screened, after which the fine portion obtained can be introduced to a certain extent in the furnace via a cellular electrode system. The operating temperature in the furance is ca. 2000.degree. C. The carbide is tapped off into pans and allowed to solidify before being crushed, screened and packed. The technical carbide contains ca. 78% CaC.sub.2, 15% CaO and some other oxides deriving from the lime and carbon material.
The energy consumption is about 3500 kWh per ton technical carbide, calculated as furnance energy. To this must be added about 300 kWh per ton in the form of auxiliary energy.
A big drawback with this known process is that the raw material must be in piece form, thus limiting the materials available. Fine grained lime/limestone and carbon materials are generally considerably cheaper than material in piece form.
The electric arc furnace method is also sensitive to the electrical properties of the raw materials, and this may also limit the choice. It is therefore a matter of importance to find a process which is insensitive to the choice of raw materials and where the raw materials are dissociated from the electrical conductivity of the furnace charge.